(1) Field of the Invention
The present invention relates to a process and apparatus for the ashing treatment of an organic substance coating film on a material to be treated, for example, a semiconductor wafer, by a plasma.
(2) Description of the Related Art
In the processing of a wafer of a semiconductor device, the basic operations of formation of a film, formation of a resist pattern, etching using the resist as a mask, and removal of the resist are repeated. As the resist-removing method in the above-mentioned cycle, a method is adopted in which the resist is ashed by using a plasma, and in this method, the resist must be removed efficiently without damage to the surface of the semiconductor wafer.
The conventional ashing treatment apparatus using a plasma is shown in FIG. 5. This ashing treatment apparatus is an oxygen down-stream type apparatus in which an oxygen plasma is applied to a semiconductor wafer.
A heating stage 14 on which a semiconductor wafer 12 to be treated is placed is arranged in an ashing treatment chamber 10 for effecting the ashing treatment, a gas-introducing opening 18 is formed in a plasma-generating chamber 16 for generating a plasma, to introduce an oxygen-containing gas to be converted to a plasma, and a microwave waveguide 22 is disposed so as to communicate with the chamber 16 through a quartz plate 20. An electroconductive plate 24 having many through holes, called a "shower plate", is arranged between the plasma-generating chamber 16 and the ashing treatment chamber 10, and this shower plate 24 is earthed (i.e. electrically grounded) to capture charged particles having a high energy, such as charged active species and electrons, in the plasma generated in the plasma-generating chamber 16, and allow a transmission therethrough of neutral active seeds which are electrically neutral and have a low energy. If the plasma is directly applied, the surface of the semiconductor wafer 12 will be severely damaged by charged particles having a high energy, and then the shower plate 24 is provided to eliminate this disadvantage. An exhaust opening 26 is arranged in the ashing treatment chamber 12 to discharge the gas formed by ashing.
When the ashing treatment is carried out by using this ashing treatment apparatus, the semiconductor wafer 12 to be subjected to the ashing treatment, and to which an organic coating film such as a resist is adhered, is placed on the heating stage 14 and heated. The oxygen-containing gas to be converted to a plasma is introduced into the plasma-generating chamber 16 from the gas-introducing opening 18, and simultaneously, a microwave of 2.45.+-.0.1 GHz is applied to the microwave waveguide 22. The microwave is transmitted through the quartz plate 20 and acts on the gas in the plasma-generating chamber 16 to generate a plasma, and the generated plasma flows into the ashing treatment chamber 10 through the shower plate 24. The charged particles in the plasma are captured by the shower plate 24 and only the neutral active species which are not charged permeate through the shower plate 24 and impinge against the semiconductor wafer 12 on the heating stage 14. Accordingly, the shower plate 24 acts as a plasma-transmitting plate for capturing the charged particles in the plasma generated in the plasma-generating chamber 16 and allowing a transmission of the neutral active seeds. When the neutral active species impinge against the semiconductor wafer 12, mainly the neutral active seeds of oxygen oxidize and ash the organic substance coating film, for example, the resist, on the semiconductor wafer 12, and the gas formed by ashing is discharged from the exhaust opening 26.
The results of the measurement of the ashing characteristics obtained by the conventional ashing treatment apparatus are shown in FIG. 6.
A resist (OFPR800 supplied by Tokyo Oka) of a novolak resin having a thickness of 1.5 .mu.m, and which is formed, by coating on a semiconductor wafer 12 having a thickness of 4 inches, is ashed under the following conditions. Namely, the output of the microwave is 1.5 kW, the pressure in the ashing treatment chamber 10 is 0.8 Torr, the oxygen flow rate in the introduced gas is 1 SLM, and the distance between the shower plate 24 and the semiconductor wafer 12 is 35 mm. The temperature .theta.(.degree.C.) of the semiconductor wafer 12 is changed in the range of from 120.degree. to 200.degree. C., and the ashing rate (.ANG./min) is measured. FIG. 6 is an Arrhenius plot in which the ashing rate V (.ANG./min) is plotted on the ordinate and (1/T).times.10.sup.3 (.degree.K.sup.-1), in which T represents the absolute temperature (.degree.K) calculated by adding 273 to the temperature .theta. (.degree.C.), is plotted on the abscissa. From the gradient of this graph, it is seen that the activation energy at the ashing treatment is about 0.5 eV. The activation energy value of 0.5 eV is in full agreement with the value measured when the ashing treatment is effected mainly by a chemical reaction between active species of oxygen and the resist (J. M. Cook and B. W. Benson, J.Electrochem. Soc., Vol. 13, No. 12, P.2459 (1983); J. E. Spencer, R. A. Borel and A. Hoff, Electrochemical Society Extended Abstracts, Vol. 86-2, Oct. 19-24 (1986), Abstract No. 285).
As apparent from the graph of FIG. 6, in the ashing treatment of the conventional ashing treatment apparatus, the ashing rate greatly depends on the temperature of the semiconductor wafer 12. More specifically, when the temperature of the semiconductor wafer 12 is 200.degree. C., the ashing rate is 20.times.10.sup.3 (.ANG./min), but as the temperature is lowered, the ashing rate is drastically reduced and when the temperature is 120.degree. C., the ashing rate is about 3.8.times.10.sup.3 (.ANG./min), i.e., lower than 1/4 of the ashing rate attained at 200.degree. C. It is difficult to keep the temperature of the semiconductor wafer 12 constant, and the ashing rate is greatly changed by a slight change of the temperature, and thus it is difficult to obtain a stable ashing rate. This temperature dependency however, is peculiar to the chemical reaction of the ashing treatment, and therefore, it has been considered that, to moderate this temperature dependency, it is necessary to change the reaction per se of the ashing treatment by changing the resist or the introduced gas, but it is difficult to obtain an improvement by this moderation.
In view of the plasma contamination, preferably the ashing treatment is carried out at a lower temperature, but according to the conventional ashing treatment, especially at low temperature, the ashing rate is low and a long time is required for completion of the ashing treatment and, therefore, the conventional ashing treatment has the problem of low productivity.